Heard of these car rental companies?

Has anyone heard of or had any experiences with the following car rental companies:

Fox Rent A Car
Advantage Rent A Car
Payless Car Rental

They seem to be quite a bit cheaper than the normal ones you see at airports (e.g. Budget, Thrifty, Dollar, etc.)  I'm planning to rent a car at Denver Airport on Sunday for a last minute whitewater rafting trip, but the prices are outrageous for some reason in Denver.  I saw these 3 companies offering substantially lower prices, and was curious if there was a catch to renting with them.  Are there a ton of hidden charges?  Are they reputable?  Any experiences/knowledge you guys have would be greatly appreciated.  Thanks.

og mike you have been around awhile but never heard of these companies?? 

I did Payless in Vegas...they did try to upsell me on a lot, just hold firm with what you book, and you will be fine

Used Payless twice. Vehicles are older than the majors, but I had no problems with them.

Good info. 4 wheels, some breaks and a steering wheel ....all I need.

When I rent a car, I prefer brakes. In gambling, I need breaks.

Thanks guys for the info, had never heard of these guys before, will probably look at Payless then.

Kelly:  I actually travel quite a bit, but hardly ever been in the position to worry about a rental car.  Anytime we go abroad, we never rented a car, and those companies probably aren't over there.  And when my lady and I rent here in the US, she always books through National/Alamo for free b/c she rents cars so much through work, that she has a ton of points built up.  First time I've gone somewhere in a while without the lady AND have needed to rent a car.  Last time, I believe was in 2003, when some buddies and I took a roadtrip to watch OU in Tuscaloosa.